In recent years, photonic integrated elements fabricated on an inexpensive Si substrate having a large area have been attracting attention. Si is a medium that is transparent for optical signals in a 1.3 μm band or in a 1.55 μm band that have been used for conventional optical communication. Various types of optical elements based on a silicon photonic wire waveguide technology for high-level optical confinement with low loss using a high-level process technology have been proposed and demonstrated.
In order to increase the transmission capacity in a silicon photonic integrated circuit, a wavelength division multiplex (WDM) silicon photonic integrated circuit, to which a WDM transmission system used for optical fiber communication is applied and where a number of optical wavelength signals that have been independently modulated are multiplexed within a silicon device for transmission and detection, is regarded to be promising.
WDM signal light that has propagated through an optical fiber, which is a transmission path, is inputted into a light receiving device in a random polarization state where S polarized waves and P polarized waves are mixed, and therefore, the light receiving device is required to have such a configuration where wavelength demultiplexing and light detection can be performed at a constant efficiency irrelevant of the state of the polarization. Therefore, a polarization splitting grating coupler through which input light of the two types of polarization can be coupled with a silicon waveguide at high efficiency is used without using a particular fabrication process.
FIG. 16 is a schematic diagram illustrating the configuration of a conventional wavelength division multiplexing optical receiver, which is herein used for the description of an example where WDM signal light in which optical signals of four wavelengths are multiplexed is received so that the respective wavelength components are separated in a wavelength demultiplexer (DEMUX) within an element so as to be converted to electrical signals in different photodetectors. WDM signal light that has entered from an optical fiber 73 is separated into S polarized waves (x) and P polarized waves (1−x) by a polarization splitting grating coupler 61 so as to be outputted. The polarization splitting grating coupler 61 has the functions of coupling an S polarized wave component of which the electrical field is perpendicular to the entrance plane and a P polarized wave component of which the electrical field is parallel to the entrance plane of WDM signal light that has entered in the vertical direction with different Si photonic wire waveguides 62 and 63 as in a TE mode (waveguide mode where the electrical field is parallel to the Si substrate) and outputting the resulting signal light.
Therefore, the output from the polarization splitting grating coupler 61 is separated into the respective wavelengths by a pair of wavelength demultiplexers 68 and 69 that correspond to the respective polarized wave components through the Si photonic wire waveguides 62, 63, 66 and 67. The signal lights that have been separated for the respective wavelengths are received by a photodiode array 72 where bidirectional input type photodiodes 721 through 724 are in an array through Si photonic wire waveguides 701 through 714, and as a result of this, a so-called polarization diversity configuration is adopted.
This configuration makes stable wavelength separation and light detection possible even when the state of the polarization of WDM signal light fluctuates within the optical fiber 73. Incidentally, in some cases, optical loss is large in the transmission path and within the integrated transmitter/receiver, and the intensity of light inputted into the photodetectors is insufficient in a large capacity WDM optical link to which the wavelength division multiplexing optical receiver 60 is applied. In such a case, it has been proposed that in order to collectively amplify WDM signal light, polarization dependent SOAs 64 and 65 should be arranged on the reception side as illustrated in FIG. 16 as a system for compensating loss with low power instead of excessively increasing the optical output of the laser that generates WDM signal light (see Patent Literature 1).
Patent Literature 1: International Publication Pamphlet No. WO2013/179467